The invention relates generally to the field of strain gages, and more particularly to transducers for accurately measuring low level forces.
Low level force transducers are used in a number of devices to measure small forces which are generated during operation to ensure that the forces remain within predetermined ranges. For example, a device for winding magnetic tape onto a reel typically includes a low level tension measuring transducer to monitor the tension on the tape as it is being wound. This helps assure not only that tape is wound onto the reel with a uniform tension, but also provides a means for helping prevent tape breakage, which happens if the forces get too large.
Typically, a low level strain gage transducer comprises a strain member which deforms as force is applied. An electrical bridge circuit with four or more arms is mounted on the strain member, and each of the arms includes an electrical resistance strain gage. When force is applied, the member and one or more of the strain gages on it are deformed slightly. This causes variations in resistance in one or more of the arms, unbalancing the electrical bridge. The amount of the unbalancing of the bridge determines the degree of deformation of the strain member, from which (along with the member's materials and design), the amount of the strain can be determined.
A typical transducer for a low level force transducer includes a housing in which the strain member is mounted. The strain member itself includes two parallel end members connected in a cantilevered fashion by two perpendicular beams. The bridge circuit is mounted on the beams. One of the end members is fixed the housing, while the other end member is free. In operation, the element to be monitored (e.g., a roller) is attached to the free end member so that force is applied to it. Since the housing and the fixed end member attached thereto are stationary, the force applied to the free end member deforms the perpendicular beams connecting the free end member to the fixed end member. This results in a change in the electrical characteristics of the bridge circuit on the beams.
The strain gage transducer is optimally positioned so that the force to be measured is applied perpendicularly to the free end member. Under those circumstances, the free end member transmits this force evenly to the beams. This results in pure tension on or compression of the beams, and the force measurement is an accurate one. If, however, the transducer is misaligned with the force being applied, that is, if it is positioned so that the force is not entirely perpendicular to the free end member (or not entirely parallel to the beams), the applied force generates complex bending forces on the beams, resulting in errors in the measurement. As a result, there is a need for a low level strain gage transducer which is not susceptible to such errors.